1. Field of the Invention
The present invention relates to a method for producing a coating film comprising hydroxyapatite, or a mixture containing hydroxyapatite and a hydroxyapatite precursor, on the surface of a substrate by use of two or three specific kinds of solutions, under specific conditions in accordance with needs. In the present invention, a coating film of hydroxyapatite having excellent bioactivity is formed on the surface of any of a variety of substrates, and the coated objects are applied to a variety of medical equipment, medical materials, etc. Also, a hydroxyapatite coating film is formed on the surface of fiber constituting a substrate such as woven fabric or non-woven fabric, to thereby serve as a mask or any of a variety of filter materials.
2. Background Art
With regard to methods for forming a hydroxyapatite coating film, there are known a plasma spraying method described in Japanese Patent Application Laid-Open (kokai) Nos. 62-34559, 62-57548, and 63-160663; a method described in Japanese Patent Application Laid-Open (kokai) Nos. 62-231669, 63-24952, and 63-46165 in which a solution or a compound containing calcium and phosphorus is applied to the surface of a substrate and the coating is sintered; etc. There are also known a sputtering method described in Japanese Patent Application Laid-Open (kokai) No. 58-109049; a flame spraying method described in the Proceedings of the First Autumn Symposium 1988 of Japan Ceramic Association, p.p. 401-402; an electrophoresis method described in the Proceedings of the First Autumn Symposium 1988 of Japan Ceramic Association, p.p. 417-418; etc.
Furthermore, there is also proposed a method which mimics a mechanism of osteogenesis in organisms, which method comprises the steps of introducing on the surface of a substrate sites for inducing formation of hydroxyapatite nuclei and soaking the substrate in simulated body fluid to grow the nuclei. Examples of the method for introducing sites inducing formation of hydroxyapatite nuclei include methods employing bioactive glass described in Japanese Patent Application Laid-Open (kokai) Nos. 4-141177 and 6-293506 and Japanese Patent Publication (kokoku) Nos. 6-29126 and 7-24686 and a method in which a substrate is phosphorylated described in Japanese Patent Application Laid-Open (kokai) No. 8-260348.
However, a method requiring high-temperature treatment, such as a plasma spraying method, is difficult to apply to substrates having poor heat resistance that comprise, for example, an organic polymer material. There is also a problem that formed hydroxyapatite is not quite the same as the hydroxyapatite found in organisms. In contrast, a method which mimics a mechanism of osteogenesis in organisms enables formation of an apatite coating film approximately similar to that found in organisms, on a substrate comprising a material having poor heat resistance such as an organic polymer material as well as a material having high heat resistance such as ceramics. However, in a method in which bioactive glass is employed so as to introduce sites for inducing apatite nuclei on the surface of a substrate, particulate glass must be prepared by melting CaO--SiO.sub.2 glass, crushing, and classifying. A method in which a substrate is phosphorylated requires intricate operations, i.e., phosphorylation of a substrate and partial post-hydrolysis.
In this connection, the present inventors have previously proposed a method for forming a hydroxyapatite coating film without requiring these intricate operations. Briefly, the method comprises the following steps: soaking a substrate in an aqueous solution containing at least calcium and phosphorus; removing the substrate from the aqueous solution and drying the substrate; soaking the dried substrate in an aqueous solution in which a hydroxyapatite component is dissolved at a substantially saturated or supersaturated concentration, to thereby form a hydroxyapatite coating film on the surface of a substrate. The method enables deposition of hydroxyapatite through simple operations. However, depending on the type of a substrate or in the case of a substrate having a large size, hydroxyapatite might be deposited on the surface to an insufficient concentration or a heterogeneous thickness. This is considered to be attributable to failure to attain homogeneous deposition of a calcium phosphate compound serving as a precursor of hydroxyapatite on the surface of the substrate.